Please refer to FIG. 8, which is a schematic view of an embodiment of a bulk acoustic wave resonator of the conventional technology. The resonator comprises a substrate 90, a bottom electrode 91, a piezoelectric layer 92, a top electrode 93, a cavity 94 and an annular piezoelectric layer recess 95. The bottom electrode 91 is formed on the substrate 90. The piezoelectric layer 92 is formed on the bottom electrode 91. The top electrode 93 is formed on the piezoelectric layer 92. The cavity 94 is formed under the bottom electrode 91 on the substrate 90. The overlapping of the top electrode 93, the piezoelectric layer 92 and the bottom electrode 91 form a resonance film of the bulk acoustic wave resonator. The annular piezoelectric layer recess 95 is formed by removing the material of the piezoelectric layer 92 around the periphery of the resonance film of the bulk acoustic wave resonator. By forming the annular piezoelectric layer recess 95, the boundary condition of the periphery of the resonance film of the bulk acoustic wave resonator is changed. Since the boundary condition of the periphery of the resonance film of the bulk acoustic wave resonator is changed, the ratio of the reflection acoustic wave and the incident acoustic wave is also changed when then incident acoustic wave reflects at the periphery of the resonance film of the bulk acoustic wave resonator. By designing and adjusting a proper width and a proper depth of the annular piezoelectric layer recess 95, the ratio of the reflection acoustic wave and the incident acoustic wave may be adjusted such that the Q factor of the bulk acoustic wave resonator is enhanced.
Since a width of the resonance film of the bulk acoustic wave resonator is usually much greater than a depth of the cavity 94, and furthermore the resonance film of the bulk acoustic wave resonator is formed by the top electrode 93, the piezoelectric layer 92 and the bottom electrode 91, especially the top electrode 93 and the bottom electrode 91 are formed by metal, therefore, the resonance film of the bulk acoustic wave resonator may be bended downwardly when affected by stress. Hence, a bottom of the bottom electrode 91 is possibly touched with the substrate 90 (a bottom of the cavity 94) such that the characteristics of the bulk acoustic wave resonator may be affected. Removing the material of the piezoelectric layer 92 around the periphery of the resonance film of the bulk acoustic wave resonator to form the annular piezoelectric layer recess 95 may affect the mechanical structure strength of the resonance film of the bulk acoustic wave resonator such that the resonance film of the bulk acoustic wave resonator become more easily bended downwardly when affected by stress. Moreover, the insufficient mechanical strength of the resonance film of the bulk acoustic wave resonator may even result in collapse of the resonance film of the bulk acoustic wave resonator.
Since the acoustic wave is propagating and resonating in the resonance film of the bulk acoustic wave resonator, therefore whether the entire flatness of the top electrode 93, the piezoelectric layer 92 and the bottom electrode 91 of the resonance film of the bulk acoustic wave resonator may directly affect the characteristics of the bulk acoustic wave resonator. In another embodiment of a bulk acoustic wave resonator of the conventional technology, a protruding structure is formed along an edge of a top surface of the bottom electrode 91. By forming the protruding structure, the boundary condition of the periphery of the resonance film of the bulk acoustic wave resonator is changed such that the ratio of the reflection acoustic wave and the incident acoustic wave is also changed. By designing and adjusting a proper dimension of the protruding structure, the ratio of the reflection acoustic wave and the incident acoustic wave may be adjusted such that the Q factor of the bulk acoustic wave resonator is enhanced. However forming the protruding structure along the edge of the top surface of the bottom electrode 91, the flatness of the piezoelectric layer 92 may be poor such that the entire flatness of the resonance film of the bulk acoustic wave resonator is affected. Hence, the characteristics of the acoustic wave propagating in the resonance film of the bulk acoustic wave resonator may be affected so that the characteristics of the bulk acoustic wave resonator are adversely affected.
Accordingly, the present invention has developed a new design which may avoid the above mentioned drawbacks, may significantly enhance the performance of the devices and may take into account economic considerations. Therefore, the present invention then has been invented.